I specifically bought my tablet to watch films when flying long distances, so for me a 10 hr battery life is crucial.
Another use case is the long weekend camping, when I won't have any power for 72 hours. It would be nice if it lasts until I get home, with some usage each day.
Admittedly, these aren't every day use cases. Typically, I use it on the tube to and from work, it sits in a dock on my desk playing music at work, similar at home, and then to bed to read a book, or I use it as the remote control for my TV.
However, if it didn't have the long battery life, I probably wouldn't have bought it in the first place. It's like buying watches, do you get the water resistant watch, the 5m waterproof watch, or the 50m divers watch? I would hazard that the majority of people buying the divers watch do not spend all day diving, but would like the opportunity of doing so.